I feel that over time the seating issue on PDL has become muddied/muddled in some quarters. To be clear, the trains concerned were/are ONLY the 1Pxx fast services, taking, give or take one & half hours end to end. The 2Pxx slower services, be they to Haslemere or Portsmouth were always planned for 450, and I would imagine are likely to remain so.
I see only one option;
a) The status quo (a mix of 444 and 450) will remain as now, and ditto going forward even/if/when the timetable in revised.
The risk to the timetable and overall performance with running 12 car 458 is I believe such, that I very much doubt it will be sanctioned in any quarter. It would put too much risk (potential disruption wise) in to the plan, not to mention additional restrictions/stress on the signallers concerned, particularly during disruption when those few (12 car 458 suitable) platforms, can never be guaranteed to be kept clear just for those 1Pxx trains formed as such.
The logistics/expense of moving/re-siting numerous signal gantries renders this a non starter.